1. Field of the Invention
This invention relates to an electroless nickel plating solution having improved stability and plating properties characterized by the addition of a stabilizer comprisig a source of gallium.
2. Description of the Prior Art
Electroless metal deposition refers to the chemical plating of a metal over an active or activated surface by chemical reduction in the absence of an external electric source. Processes and compositions useful therefor are known, are in substantial commercial use and are described in numerous publications. Electroless nickel formulations are disclosed in U.S. Pat. Nos. 3,661,596; 3,738,849; 3,915,713; and 4,038,085, all included herein by reference.
Known electroless nickel plating solutions generally comprise four major ingredients dissolved in water. They are (1) a source of plating metal ions, (2) a reducing agent such as hypophosphite or boron compound such as borohydride or an amine borane, (3) a pH adjuster to provide required solution pH and, preferably, (4) a complexing agent for the plating metal ions sufficient to prevent their precipitation from solution.
In addition to the above major components, it is known that other additives are needed for a practically useful formulation, for example, to stabilize the solution and improve deposit properties. With regard to stabilization, it is known that certain additives added to an eletroless solution in properly controlled trace quantities act as stabilizers and retard the rate of bath decomposition. Generally, these additives, or stabilizers as they are referred to in the art, are catalytic poisons when used in excess of minute quantities. The concentration of the stabilizer in solution is usually critical. Trace quantities, typically in the range of a few parts per million, provide stability. An excess of stabilizer may partially or totally stop deposition of the electroless nickel. Known stabilizers are disclosed in the above referenced patents and others are disclosed in U.S. Pat. Nos. 3,717,482 and 3,719,508, also incorporated herein by reference.